Process for producing a calcium ferrite for making steels

ABSTRACT

A process for producing a calcium ferrite for making steels by mixing an iron powder ore and a limestone powder so that the mol ratio of CaO/Fe2O3 is 0.5 to 6, granulating the mixture, then covering the granules with the limestone powder and firing them at a temperature above 1,100* C.

United States Patent 1151 3,649,248

Ishimitsu et a1. 5] Mar. 14, 1972 [54] PROCESS FOR PRODUCING A 3,519,386 7/1970 Fedock et al. ..23/51 CALCIUM FERRITE FOR MAKING 3,519,471 7/1970 Ban ..75/94 X 2,127,632 8/1938 Najarian ..75/3 STEELS 2,159,977 5/1939 Nicholas 75/94 X [72] Inventors: Akitoshi lshimilsu; Takeo Furui; Katsu- 2,869,850 l/l959 Wienert ..75/3 hiko Sam; Yasumasa Sawam all f 3,163,519 12/1964 Hanson et a1. ....-75/3 Kitakyushu, Japan 3,313,617 4/1967 Ban et al ..75/5

[73] Assignee: Yawata Iron & Steel Co., Ltd., Tokyo, FOREIGN PATENTS OR APPLICATIONS Japan 45,744 5/1939 Netherlands ..75/94 [22] Filed: Oct. 28, 1968 Primary ExaminerI'Ierbert T. Carter [21] Appl' 771020 Attorney-Wenderoth, Lind & Ponack 52 US. (:1 ..75/94, 75/3, 75/5, ABSTRACT 1 17/100, 117/ 169 R, 23/51 R, 23/3 A process for producing a calcium ferrite for making steels by [51] Int. Cl. ..C22b 9/10 mixing an iron powder ore and a limestone powdcr so that the [58] F1eld of Search ..23/51, 313; 75/3, 5, 93 F, mo] ratio f g g/ 0 is 5 to 6 granulating the mixture,

117/ 100, 169 R then covering the granules with the limestone powder and firing them at a temperature above 1,100 C. [5 6] References Cited 5 Claims, 2 Drawing Figures UNITED STATES PATENTS A 3,169,852 2/1965 Price .75 94 PATENTEDMAR 14 m2 3,649,248

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Powderedironore Powdered lineslone Additives CaOCa(0H)2- Weighing Weighing Weighing Mixing Pulveringalion hloisiure regulalion Granulation Powdered limestone Drying Calcined lime or limeslone Firing Cooling alcinedlime Slevlng Produci INVENTORS Akiroshi Ishimilsu Takeo Furui Katsuhiko Soto Yasumasa Sawamura ATTORNEY PROCESS FOR PRODUCING A CALCIUM FERRITE FOR MAKING STEELS BACKGROUND OF THE INVENTION FIELD OF INVENTION This invention relates to a process for producing calcium ferrite agglomerates to be used as fluxes for making steel, wherein an iron-containing powder ore and a limestone powder are mixed in such a way that the mol ratio of CaO/Fe,0= is 0.5 to 6, the mixture is granulated. Thereupon the obtained granules are sprinkled with a limestone powder or quicklime powder in the intergranular gaps and the mixture is then fired at a temperature above 1,l C. in a rotary kiln or shaft furnace.

DESCRIPTION OF THE PRIOR ART Heretofore, quicklime has been generally used as a slagging agent in the case of producing a low phosphorus steel in a steel-refining converter. However, with quicklime the slagging is not always quick enough and thereby also the dephosphorizing process is heavily influenced. That is, for example, in an oxygen top-blowing converter, quicklime is fed substantially in the middle stage of a carbon blowing period to proceed to the subsequent dephosphorizing process. In this case, if a slagging agent high in slagging velocity is added, the dephosphorizing velocity would, of course, be increased and the lime addition would be very effectively utilized, whereby a time required for making steel could largely be shortened.

Therefore, an addition of lime, for example, in the form of calcium ferrite into the bath during the refining process of steel has been studied. This method has been recognized to be very effective in facilitating the slagging and improving the dephosphorizing velocity, when a molten agglomerate of calcium ferrite was prepared by melting iron ore and limestone in an electric furnace and applied onto the steel both in a converter. However, this method could not be practically adopted, because it had an economic difficulty on account of large electric power being required in preparing the same in the electric furnace.

Therefore, it is absolutely necessary in order to be able to use calcium ferrite as a slagging agent for refining steel that calcium ferrite agglomerate should be cheap and, in respect of the composition, the sulphur content should be less than 0.05 percent at most. Further, when applying it to practical use, for example, in an oxygen top-blowing converter it is desired that granules of calcium ferrite be so coarse as not to be blown off by rising gases and be of uniform size.

There has been already proposed a process for producing such calcium ferrite, wherein limestone is fired in a rotary kiln to make quicklime therefrom, onto which iron oxide powder is sprayed and thereafter the quicklime thus covered with iron oxide powder is again fired so as to make only the surface thereof calcium ferrite or a process, wherein limestone and iron-containing powder are mixed together and the mixture is sintered with an addition of coke.

However, these methods have disadvantages, when products of these methods are used in a converter or the like that in the former case the slagging velocity is low on account of the melting point being high, and in the latter case effloresce is easily caused on account of the surface area being large, because only the surface of quicklime or the sintered surface area becomes calcium ferrite respectively.

SUMMARY OF THE INVENTION The present invention is to solve such problems as are mentioned above.

That is, the main object of the present invention is to provide the production of calcium ferrite of excellent quality for use in steelmaking which characterized by a high slagging velocity, no efiloresce being caused and no danger of floating up together with rising gases on account of its considerable weight.

Further, the present invention has for its object the production of calcium ferrite having low content in sulfur, as being produced by firing iron-containing powder ore of a good quality low in the content of sulfur, phosphorus and SiO, and limestone as raw materials in an atmosphere containing no sulfur.

An another object of the present invention is to provide a very economical method of producing calcium ferrite of excellent quality, as iron-containing powder ore as, for example, hematite powder and limestone are mixed in small granules of less than about 0.1mm., which makes the reaction at a relatively low temperature possible.

A further object of the present invention is to provide the production of calcium ferrite of excellent quality, which is characterized by easy handling when used in refining steels, as the raw materials are well mixed, then molded and covered with lime powder and fired, whereby the produced granules are arranged at a proper granularity.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings shall be briefly explained.

FIG. 1 is a CaOFe,O system phase diagram.

FIG. 2 shows a flow sheet of steps for working the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention shall be concretely described in the following.

The inventors of the present invention made experiments in manufacturing calcium ferrite, in which an iron-containing powdered ore and limestone were mixed in predetermined ratios so that CaO'Fe O or 2CaO-Fe O which is a typical composition of calcium ferrite, may be contained in a single or compound form, then the mixture was made into granules having an average diameter of l3mm., which were then fired in a test rotary kiln having a length of 600cm. and an inside diameter of 40cm. at a temperature of l,200 to l,250 C. The results of the experiments showed that, as shown in FIG. 1 indicating the CaOFe,O system equilibrium phase diagram, rings of the molten charge were produced in the rotary kiln or large lumps were produced by including therein unreacted raw materials, because of the low melting point of the calcium ferrite having the above-mentioned composition i.e., about 1 ,200 C.

Further, when a shaft furnace was used, hangings were produced, resulting in a remarkable deterioration of the working rate.

It was also found that in either case of employing the rotary kiln or a shaft furnace, when the firing was carried out in such a low temperature range as below 1,200 C. that no melt of the calcium ferrite was produced, the reaction for producing the calcium ferrite took a long time and was hardly practical. As results of various investigations made to prevent fusing between granules in making a calcium ferrite of properties as above-mentioned, the inventors of the present invention have succeeded in obtaining agglomerates of calcium ferrite in the form of pellets and the like of a good quality by sprinkling the granulates with a limestone powder or quicklime powder and interposing excessive limestone powder or the like in the intergranular gaps so that the granules may be prevented from being fused together or there may be eliminated such troubles in the operation as of a molten charge being stuck onto the furnace wall of the kiln or large lumps being produced within the kiln on account of the fusing of a charge.

In order to economically produce a calcium ferrite, the reaction for producing the calcium ferrite must be completed within a time as short as possible, but for this purpose, a high temperature above l,200 C. is requested. Further, as granules are to be covered on their surfaces with a limestone powder as a heatproofing agent before subjected to the firing, in view of troubles of the fusion of the charge and the like being caused during the firing as above-mentioned, there are obtained by firing the granules two-layer pellets, each of which having the inner core part consisting of limestone and powdered iron source and the outer shell consisting of quicklime.

Practically in the experiments made by the inventors the pellets were obtained by firing the granules at l,250 C. at 0.6 r.p.m. for 30 minutes in an oxidizing atmosphere and causing them to remain in a rotary kiln for about 120 minutes. It was recognized that there could be produced thereby pellets consisting of a calcium ferrite of good quality, not fused to each other and not accompanied by such a phenomenon as of being stuck on the furnace wall.

Further, as the portions of limestone added other than those adhered to the surfaces of the raw pellets or those which are to become the outer shell of the pellets formed by the reaction during the firing are to be discharged out of the rotary kiln in the powdery or small granular form of calcined lime together with the fired pellets, they are easily separated from said pellets by a simple sieving operation.

The thus obtained calcined lime powder can circularly be used in the firing kiln for the purpose of preventing the pellets from being fused together.

FIG. 1 is a CaOFe O phase diagram in producing calcium ferrite of the present invention, wherein the abscissa shows the content of Fe O in percentage and the ordinate and melting point. As is evident from this diagram, the melting point declines at a content of Fe O of about 60 percent on and reaches the lowest point at about 80 percent thereof. Though it is difficult to judge the acceleration of slagging velocity during the process of steelmaking only from the above-discussed melting curve, because of influence by other conditions such as, for example, composition, it is desirable to employ CaO-Fe OBS, which is a slagging agent of low melting point, that is, a slagging agent which contains Fe O in a relatively large amount. However, 80 percent is the maximum, because,

as above-mentioned, after 80 percent the melting point takes a sharp rising tendency and it suffers from a shortage of CaO source.

EXAMPLE 1 According to the above-described process of the present invention, a calcium ferrite was produced by using a high-quality hematite powder and a limestone powder each of a granularity of less than 0. l mm. in a rotary kiln of a length of 600cm. and an inside diameter of 40cm. at 0.6 rpm.

The mixing rates of the hematite powder and limestone powder were respectively 65 and 35 percent and the mol ratio of CaO/Fe O was 0.86. The composition in this case was of a melting point intermediate between those of a hemicalcium ferrite and monocalcium ferrite, that is, about 1 ,200 C. and of becoming a softened state already at l 150 C. After the intergranular gaps were filled with lime powder in an amount about twice as large as the amount of the pellets in order to prevent the pellets from being fused together and the pellets were roasted for 120 minutes at a firing temperature of L200 C, corresponding to the maximum temperature in the kiln,

whereby excellent pellets of the calcium ferrite could be obtained. in case the roasting temperature was further elevated to l,250 C, the roasting was sufficient with 90 minutes.

EXAMPLE 2 in case the mol ratio of CaO/FqO was 1.60, the composition was intermediate between those of a di-calcium ferrite and a monocalcium ferrite. The raw pellets were covered with a limestone powder in an amount substantially the same as of the raw pellets to charge the intergranular gaps and were fired at L200 C. for 120 minutes and l,250 C. for 90 minutes to produce excellent pellets of the calcium ferrite.

EXAMPLE 3 When the mol ratio of CaO/Fe O was 2.97, the temperature, at which a liquid phase began to exist, was above l,400 C. Therefore, below 1,250 C, there occurred no fusion of the raw pellets. The strength of the fired pellets was about 190 kgJpellet in the case of 1,250 C. Judging from the fact that the pellets were pulverized in 6 days due to effloresce, a large amount of unreacted CaO was thought to be contained therein. However, as those which can stand the storage in the field for l to 2 days are already practically enough for use in a steelmaking plant, this fired product can be said to be an excellent flux.

EXAMPLE 4 In case the mol ratio of CaO/Fe,0, was 6, the amount of CaO meant such an excessiveness that a sprinkling of limestone powder onto intergranular gaps for the purpose of preventing a fusion of granules was dispensable, if raw pellets should be fired at 1,250" C. and 1.300 C., and the fired pellets obtained in this example showed strengths of and 183 kg./pellet and did not efiloresce for 3 days.

In each of the above-mentioned examples there could be obtained products, which were especially easy in slagging and contained only less than 0.05 percent sulfur.

As is clearly seen from the above examples, when the mol ratio of CaO/Fe O is low, limestone powder is required in a larger amount to be sprinkled onto intergranular gaps to prevent the fusion of granules on account of the melting point of the fired product being low. However, less is required as the mol ratio increases, and in the latter case the composition becomes 2CaO+2Ca0Fe O and the above-mentioned twolayer calcium ferrite is not fired. Therefore, in case the mol ratio exceeds 6, the obtained calcium ferrite pellets are not well fitted to a flux for use in steelmaking, because in this case the firing temperature must be elevated and the product is likely to be effloresced.

As above-mentioned, the present invention is to provide a process for producing excellent calcium ferrite pellets for use in steelmaking, said calcium ferrite consisting of two layers, wherein the central part is made of calcium ferrite and the outer peripheral part is made of CaO, or having the composition of CaO-l-2CaO'Fe O by mixing powdered iron ore and lime stone powder in such a ratio as the mol ratio of CaO/Fe O may be in the range of 0.5 to 6, granulating the mixture into pellets, sprinkling limestone powder onto intergranular gaps and then firing the pellets at a temperature above l,l00 C, and the products thus obtained by this process display various advantageous properties such as being so high in the slagging velocity as to be able to largely reduce the time required for steelmaking, low in the content of sulfur series impurities, very convenient in handling such as transportation or charging a kiln and sufficiently durable as is required by a steel manufacturing plant.

We claim:

1. A process for producing a calcium ferrite agglomerate consisting of two layers wherein the inner layer is made of calcium ferrite and the outer layer is made of CaO consisting essentially of mixing an iron-containing powdered ore and a limestone powder together so that the mol ratio of cao/Fep, is 0.5 to 6, granulating the mixture, then covering the granules with the limestone powder and firing them at a temperature above l,l00C. to 1,300" C.

2. The process according to claim 1, wherein an iron-containing powdered ore and a limestone powder are mixed together so that the mol ratio of CaOIFe=O is 0.5 to 6, the mixture is granulated, then the granules are covered with the limestone powder and fired in a rotary kiln.

3. The process according to claim 1, wherein an iron-containing powder ore and a limestone powder are mixed together so that the mol ratio of CaO/Fe O; is 0.5 to 6, the mixture is granulated and the granules are covered with the limestone powder and fired in a vertical oven.

4. The process according to claim 1, wherein said iron-containing powder ore and limestone powder are of a granularity of less than 0. lmm.

5. The product of the process of claim 1.

6. The process according to claim 1 wherein the firing temperature is l,200 C. to 1,300 C. 

2. The process according to claim 1, wherein an iron-containing powdered ore and a limestone powder are mixed together so that the mol ratio of CaO/Fe2O3 is 0.5 to 6, the mixture is granulated, then the granules are covered with the limestone powder and fired in a rotary kiln.
 3. The process according to claim 1, wherein an iron-containing powder ore and a limestone powder are mixed together so that the mol ratio of CaO/Fe2O3 is 0.5 to 6, the mixture is granulated and the granules are covered with the limestone powder and fired in a vertical oven.
 4. The process according to claim 1, wherein said iron-containing powder ore and limestone powder are of a granularity of less than 0.1mm.
 5. The process according to claim 1 wherein the firing temperature is 1,200* C. to 1,300* C. 